1. Technical Field
The present invention relates to an oxymethylene copolymer resin composition. More particularly, the present invention is concerned with an oxymethylene copolymer resin composition comprising an oxymethylene copolymer resin and a poly-.beta.-alanine, wherein the oxymethylene copolymer resin comprises a plurality of oxymethylene copolymer chains, each comprising recurring oxymethylene monomer units and oxyalkylene monomer units inserted therein in a specific ratio, wherein the oxymethylene copolymer chains have specific terminal groups, and wherein the poly-.beta.-alanine is in a finely pulverized form. The oxymethylene copolymer resin composition of the present invention exhibits not only mechanical properties which are comparable or superior to the mechanical properties of conventional oxymethylene polymer compositions as well as conventional oxymethylene polymers, but also exhibits an excellent thermal stability which has not conventionally been achieved. The conventional oxymethylene polymers have serious disadvantages in that the unstable terminals of the oxymethylene polymer chains are susceptible to heat decomposition to form formaldehyde, and the formed formaldehyde is likely to be oxidized to thereby form formic acid. The formic acid thus formed promotes a decomposition of the main chain of the oxymethylene polymer. However, in the oxymethylene copolymer resin composition of the present invention, not only can the formation of formaldehyde which is likely to be unfavorably caused by a heat decomposition of the unstable terminals of the copolymer chains be suppressed due to the oxyalkylene monomer units inserted in the recurring oxymethylene monomer units in a specific ratio, but also the finely pulverized poly-.beta.-alanine contained in the composition efficiently captures formaldehyde which is still likely to be generated due to the inherent difficulty in complete suppression of the formation of formaldehyde, so that the formation of formic acid (which promotes a decomposition of the main chain of the oxymethylene copolymer) can be effectively suppressed. Therefore, the thermal stability of the oxymethylene copolymer resin composition of the present invention is extremely high under oxygen-containing atmosphere conditions as compared to the thermal stability of the conventional oxymethylene polymer compositions as well as the conventional oxymethylene polymers.
2. Background Art
Conventionally, oxymethylene homopolymers have been widely used as materials for automobile parts, electrical parts, etc., since the oxymethylene homopolymers have not only a good balance of mechanical properties but also excellent fatigue resistance properties. However, the thermal stability of the conventional oxymethylene homopolymer during the molding thereof is unsatisfactory. This is because the terminal acetyl groups of the oxymethylene homopolymer are likely to be eliminated from the main chain of the homopolymer by heating, and the amount of the formed formaldehyde is increased in accordance with the advance of the decomposition of the terminals of the homopolymer. When a large amount of formaldehyde is formed, disadvantages occur such that the homopolymer is likely to suffer foaming during the molding thereof and that traces of escape of gaseous formaldehyde are left in the surface of the resultant shaped article to cause the surface appearance to be poor. Further, the formed formaldehyde is oxidized by oxygen present in the molding machine to thereby form formic acid, and the formed formic acid promotes the decomposition of the main chain of the oxymethylene homopolymer.
Unexamined Japanese Patent Application Laid-Open Specification No. 5-5017 discloses an oxymethylene copolymer exhibiting not only mechanical properties which are comparable to the mechanical properties of an oxymethylene homopolymer, but also an improved thermal stability. In this prior art document, a small amount of oxyalkylene monomer is inserted in the polymer chain of the oxymethylene homopolymer, and the amount of terminal formate groups is decreased. However, although the thermal stability of the oxymethylene copolymer in a nitrogen atmosphere is improved, the thermal stability under oxygen-containing atmosphere conditions is still poor.
Unexamined Japanese Patent Application Laid-Open Specification No. 2-247247 discloses an oxymethylene polymer composition comprising an oxymethylene polymer and a poly-.beta.-alanine. In this composition, when the oxymethylene polymer is an oxymethylene homopolymer, the terminal acetyl groups are eliminated by heating in the molding of the composition, and the decomposition of the terminals of the polymer advances, so that the thermal stability of the composition under oxygen-containing atmosphere conditions is poor. On the other hand, when the oxymethylene polymer is a commercially available oxymethylene copolymer, the mechanical properties of the composition are poor.